A rare earth sintered magnet is produced by pulverizing a magnetic alloy into an alloy powder, compacting the alloy powder to obtain a green compact, sintering the green compact and then subjecting the sintered body to an aging treatment. Rare earth sintered magnets currently used extensively in various fields of applications include a samarium-cobalt (Sm—Co) type magnet and a neodymium-iron-boron type magnet (which will be herein referred to as an “R—T—(M)—B type magnet” but is also called an “R—Fe—B type magnet” normally). Among other things, the R—T—(M)—B type magnet is used more and more often in various types of electronic appliances. This is because the R—T—(M)—B type magnet exhibits a maximum energy product (BH)max that is higher than any of various other types of magnets, and yet is relatively inexpensive.
In the general formula R—T—(M)—B of the neodymium-iron-boron type magnet, R is at least one of the rare earth elements including yttrium (Y) and is typically neodymium (Nd), T is either iron (Fe) alone or a mixture of Fe and a transition metal element, M is at least one additive, and B is either boron alone or a mixture of boron and carbon. More particularly, T is preferably either Fe alone or a mixture of Fe and at least one of Ni and Co. In the latter case, Fe preferably accounts for about 50 at % or more of T. The additive M is preferably at least one element selected from the group consisting of Al, Ti, Cu, V, Cr, Ni, Ga, Zr, Nb, Mn, Mo, In, Sn, Hf, Ta and W, and preferably accounts for about 1 mass % or less of the entire magnet. Also, where B is a mixture of boron and carbon, boron preferably accounts for about 50 at % or more of the mixture. R—T—(M)—B type sintered magnets, to which various preferred embodiments of the present invention are applicable, are described in U.S. Pat. Nos. 4,770,723 and 4,792,368, for example, which are hereby incorporated by reference.
In the prior art, an R—T—(M)—B type alloy has been prepared as a material for such a magnet by an ingot casting process. In an ingot casting process, normally, rare earth metal, electrolytic iron and ferroboron alloy as respective starting materials are melted by an induction heating process, and then the melt obtained in this manner is cooled relatively slowly in a casting mold, thereby preparing an alloy ingot.
Recently, a rapid cooling process such as a strip casting process or a centrifugal casting process has attracted much attention in the art. In a rapid cooling process, a molten alloy is brought into contact with, and relatively rapidly cooled and solidified by, the outer or inner surface of a single chill roller or a twin chill roller, a rotating chill disk or a rotating cylindrical casting mold, thereby making a rapidly solidified alloy, which is thinner than an alloy ingot, from the molten alloy. The rapidly solidified alloy prepared in this manner will be herein referred to as an “alloy flake”. The alloy flake produced by such a rapid cooling process normally has a thickness of about 0.03 mm to about 10 mm. According to the rapid cooling process, the molten alloy starts to be solidified from a surface thereof that has been in contact with the surface of the chill roller. That surface of the molten alloy will be herein referred to as a “roller contact surface”. Thus, in the rapid cooling process, columnar crystals grow in the thickness direction from the roller contact surface. As a result, the rapidly solidified alloy, made by a strip casting process or any other rapid cooling process, has a structure including an R2Fe14B crystalline phase and an R-rich phase. The R2Fe14B crystalline phase usually has a minor-axis size of about 0.1 μm to about 100 μm and a major-axis size of about 5 μm to about 500 μm. On the other hand, the R-rich phase, which is a non-magnetic phase including a rare earth element R at a relatively high concentration, is dispersed in the grain boundary between the R2Fe14B crystalline phases.
Compared to an alloy made by the conventional ingot casting process or die casting process (such an alloy will be herein referred to as an “Ingot alloy”), the rapidly solidified alloy has been cooled and solidified in a shorter time (i.e., at a cooling rate of about 102° C./sec to about 104° C./sec). Accordingly, the rapidly solidified alloy has a finer structure and a smaller average crystal grain size. In addition, in the rapidly solidified alloy, the grain boundary thereof has a greater area and the R-rich phase is dispersed broadly and thinly in the grain boundary. Thus, the rapidly solidified alloy also excels in the dispersiveness of the R-rich phase. Because the rapidly solidified alloy has the above-described advantageous features, a magnet with excellent magnetic properties can be made from the rapidly solidified alloy.
An alternative alloy preparation method called “Ca reduction process (or reduction-diffusion process)” is also known in the art. This process includes the steps of adding metal calcium (Ca) and calcium chloride (CaCl) to either the mixture of at least one rare earth oxide, iron powder, pure boron powder and at least one of ferroboron powder and boron oxide at a predetermined ratio or a mixture including an alloy powder or mixed oxide of these constituent elements at a predetermined ratio, subjecting the resultant mixture to a reduction-diffusion treatment within an inert atmosphere, diluting the reactant obtained to make a slurry, and then treating the slurry with water. In this manner, a solid of an R—T—(M)—B type alloy can be obtained.
It should be noted that any small block of a solid alloy will be herein referred to as an “alloy block”. The “alloy block” may be any of various forms of solid alloys that include not only solidified alloys obtained by cooling a melt of a material alloy either slowly or rapidly (e.g., an alloy ingot prepared by the conventional ingot casting process or an alloy flake prepared by a quenching process such as a strip casting process) but also a solid alloy obtained by the Ca reduction process.
An alloy powder to be compacted is obtained by performing the steps including coarsely pulverizing an alloy block in any of these forms by a hydrogen pulverization process, for example, and/or any of various mechanical milling processes (e.g., using a feather mill, power mill or disk mill), and finely pulverizing the resultant coarse powder (with a mean particle size of about 10 μm to about 1000 μm) by a dry milling process using a jet mill, for example. The alloy powder to be compacted preferably has a mean particle size of about 1.5 μm to about 7 μm to achieve sufficient magnetic properties. It should be noted that the “mean particle sizes” of a powder herein refers to a mass median diameter (MMD) unless stated otherwise. The coarse powder may also be finely pulverized by using a ball mill or attritor.
The hydrogen pulverization process is a pulverization technique that utilizes the phenomenon that very small cracks are created in the rare earth alloy material (typically an alloy block) due to the volume expansion of the alloy material being exposed to a hydrogen gas atmosphere. This expansion is caused by the hydrogenation of the rare earth element that is included in the alloy material. Compared to the mechanical milling process, the hydrogen pulverization process increases the productivity and reduces the oxidation of the rare earth element in the subsequent processing and manufacturing steps. When a rapidly solidified alloy is used as the material alloy block, the alloy block can be coarsely pulverized by the hydrogen pulverization process to a size of about 1 mm or less (typically to a mean particle size of about 10 μm to about 1000 μm). On the other hand, where the material alloy block is an alloy ingot or a solid alloy that has been prepared by the reduction-diffusion process, the coarse powder obtained will have a mean particle size of about 1 cm.
In the prior art, the hydrogen pulverization of an R—T—(M)—B type alloy is normally performed by filling a container, made of a stainless steel such as SUS304, with rare earth material alloy blocks and then subjecting the alloy blocks to hydrogen absorption and hydrogen desorption processes inside a hydrogen furnace.
Specifically, first, the alloy blocks, stored in the container, are loaded into the hydrogen furnace, where a reduced-pressure atmosphere is created. Next, a hydrogen gas is supplied into the hydrogen furnace, thereby making the alloy blocks occlude (or absorb) hydrogen. In this hydrogen occlusion (or absorption) process, the rare earth element included in the alloy blocks is hydrogenated. The hydrogenated portions of the alloy blocks expand their volumes, thereby creating cracks there. Subsequently, after a predetermined amount of time has passed, the hydrogen gas is exhausted from the hydrogen furnace to create a reduced-pressure atmosphere inside the furnace. At the same time, the furnace is also heated to make the hydrogenated portions of the alloy blocks desorb hydrogen. Thereafter, an inert gas is introduced into the furnace, thereby cooling the resultant coarse powder. In this cooling process, to cool the coarse powder with the inert gas more efficiently, a gaseous flow may be produced inside the hydrogen furnace by a fan provided inside the hydrogen furnace. Also, to increase the efficiency of this cooling process, a container (hydrogen pulverization case) as disclosed by the applicant of the present application in U.S. Pat. No. 6,247,660 B1, which is hereby incorporated by reference, is preferably used.
In the conventional hydrogen pulverization process, however, the hydrogen furnace cannot always maintain a completely airtight condition inside it. Thus, particularly while the hydrogen furnace has a reduced pressure inside, oxygen in the air should flow into the hydrogen furnace easily. But if oxygen is present inside the hydrogen furnace, then the rare earth element is oxidized, thus deteriorating the magnetic properties of sintered magnets to be obtained. For that reason, to minimize this unwanted oxidation, the gases should be introduced into, and exhausted from, the hydrogen furnace as quickly as possible. Also, to increase the productivity, the coarse powder needs to be cooled by the inert gaseous flow in the shortest possible time.
However, in the conventional hydrogen pulverization process, if the gases are introduced or exhausted in too short a time or if the inert gas is supplied at an excessively high flow rate into the hydrogen furnace for the purpose(s) of minimizing the disadvantageous oxidation and/or increasing the cooling rate (or productivity), then the coarse powder, obtained by the hydrogen pulverization process, might be blown off and scattered inside the hydrogen furnace. The scattered powder is mostly composed of relatively small particles, which include the rare earth element at a rather high percentage. Accordingly, if these small particles are scattered, then the overall composition of the coarse powder inside the container is different from the intended or desired composition. As a result, the desired magnetic properties may not be achieved. Also, those powder particles, which have been blown off, scattered and left at various locations inside the hydrogen furnace, may be oxidized when the hydrogen furnace is opened and exposed to the air. In that case, those oxidized alloy powder particles may be mixed with a coarse powder of the next batch during the next hydrogen pulverization process. Then, a defective coarse powder like this may result in a partially incompletely sintered body (i.e., decrease in sintered density). That is to say, the scattering of those small powder particles in the hydrogen pulverization process adversely decreases the yield of the material. Furthermore, if a portion of the hydrogen furnace is made of carbon, then the amount of carbon included in the rare earth alloy material (i.e., the coarse powder) may increase, thus possibly deteriorating the magnetic properties of the resultant sintered magnets.
Nevertheless, if the gases are introduced or exhausted, or the gaseous flow is produced, at rates that are too low to cause scattering of the small powder particles, then it takes too much time to cool the coarse powder obtained, thus decreasing the throughput. In addition, since a lot of air (or oxygen) should enter the furnace, the resultant magnetic properties may deteriorate, or in the worst-case scenario, the material might ignite.
Among other things, the powder particles, obtained by subjecting a block of a rapidly solidified alloy to such a hydrogen pulverization process, are relatively fine and easily oxidizable and many of them are small powder particles that are easily scattered inside the furnace. Also, those fine powder particles are normally packed densely enough inside the container, and cannot be ventilated so easily with the inert gaseous flow. That is to say, those fine powder particles cannot be cooled so efficiently. Accordingly, to cool the fine powder particles almost as efficiently as coarse powder particles, the inert gas should be supplied at a relatively high flow rate because of this reason also. Thus, the above-described problems caused by the unintentional scattering of powder particles are particularly significant in a hydrogen pulverization process of an alloy block obtained by a rapid cooling process.
These problems arise not only in the hydrogen pulverization process of a rare earth alloy block but also in any other hydrogenation process (e.g., HDDR process carried out to prepare a powder for an anisotropic bonded magnet).